1. Field of the Invention
The present invention relates to a hydrostatic continuously variable transmission comprising a swashplate type hydraulic pump having a pump swashplate and annularly arranged pump plungers whose suction and discharge strokes are provided by the pump swashplate, a swashplate type hydraulic motor having a motor swashplate and annularly arranged motor plungers whose expansion and shrinkage strokes are provided by the motor swashplate, and a hydraulic closed circuit formed between these hydraulic pump and motor.
2. Description of the Prior Art
Hydrostatic continuously variable transmission are already known, for example, as disclosed in Japanese patent application Laid-open No. 153057/86. In the conventional hydrostatic continuously variable transmission, the middle point of a discharge region of the hydraulic pump is aligned with a tilting axis of the pump swashplate in a rotational direction of the hydraulic pump and the middle point of an expansion region of the hydraulic motor is aligned with a tilting axis of the motor swashplate in a rotational direction of the hydraulic motor. Therefore, the pump plungers and the motor plungers receive angled compression loads from the pump and motor swashplates at their substantially most projected positions, respectively and thus receive large bending moments due to lateral components of such loads, therefor. This is one of the causes for increasing frictional loss of the plungers.
Further, the present inventors have made clear that the back pressures of the hydraulic pump and the hydraulic motor are obstacles to an improvement of transmitting efficiency and have made efforts that a passage resistance in the hydraulic closed circuit is reduced to decrease such back pressures, but has reached a certain limit for the reasons of insurance of a pressure resistance in an oil passage defining portion and of compactification and have not led to the accomplishment of the intended purpose.